Support bind mounting host files, as well as directories in LXC

Support bind mounting host files, as well as directories in LXC

Currently libvirt-lxc checks to see if the destination exists and is a
directory.  If it is not a directory then the mount fails.  Since
libvirt-lxc can bind mount files on an inode, this patch is needed to
allow us to bind mount files on files.  Currently we want to bind mount
on top of /etc/machine-id, and /etc/adjtime

If the destination of the mount point does not exists, it checks if the
src is a directory and then attempts to create a directory, otherwise it
creates an empty file for the destination.  The code will then bind mount
over the destination.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>